1. Field of the Invention
The present invention relates to a crystal oscillator for stabilizing the frequency to the ambient temperature variations using a thermosensitive element and a heating device.
2. Description of the Related Art
A crystal oscillator for stabilizing the frequency to ambient temperature variations can be of well-known OCXO (oven-controlled crystal oscillator) and TCXO (temperature compensated crystal oscillator) types. The OCXO type encloses a crystal element and a peripheral circuit in a package, and the entire package is covered with a heater, thereby stabilizing the temperature of the crystal element to the ambient temperature variations and excelling in the temperature characteristic, but having the disadvantage of large power consumption and long warm-up time. On the other hand, the TCXO type is provided with an temperature characteristic compensation circuit in a peripheral circuit to compensate for an oscillation frequency based on the temperature detection value of a thermosensitive element, thereby requiring smaller power consumption and shorter warm-up time than the OCXO type, but having lower temperature stability.
Thus, a structure having better temperature characteristic and requiring no larger power consumption than the TCXO, that is, an intermediate structure between the OCXO type and the TCXO type, has been proposed (patent document 1). FIG. 1 shows a crystal resonator of a crystal oscillator described in the patent document 1. The crystal resonator portion is installed inside a package 11 formed by a base 11a and a cover body 11b. A substrate 12 is formed in the package 11, and crystal elements 13a and 13b are arranged at the positions opposite each other on one side of the substrate 12 with a clearance between them. A heater 14 is provided at the center of the substrate 12, and the heater 14 is heat controlled based on the temperature detection value of a thermosensitive element 15 mounted near the heater 14. The oscillation circuit unit is provided outside the package 11, and the oscillation circuit unit is enclosed by an external package not shown in the attached drawings. With the above-mentioned configuration the thermosensitive element 15 can detect a change of an ambient temperature, the heating value of the heater 14 can be adjusted, and the temperature of the crystal elements 13a and 13b can be stabilized with a very simplified configuration.
However, the crystal oscillator of the patent document 1 has the following problems. That is, when the ambient temperature changes, the temperature of the external package not shown in the attached drawings also changes, and then the temperature of the internal package 11 changes, thereby sequentially changing the temperatures of the structure and parts. In this case, the temperature change transmits inwards. Since the heater 14 is positioned below the center of the crystal elements 13a and 13b and near the thermosensitive element 15, and the position of the crystal elements 13a and 13b is different from the position of the oscillation circuit, the temperature difference between the crystal elements 13a and 13b and the oscillation circuit becomes large while heat flows from the side of the oscillation circuit, and is transmitted to the crystal elements 13a and 13b inside the internal package 11. In the meantime, it takes a long time to transmit the temperature change to the center, allow the temperature control effect by the heater 14 to work, and allow the effect of suppressing the temperature change to spread from the heater to the entire device.
Additionally, from the configuration shown in FIG. 1, the ambient temperature change is first received by the crystal elements 13a and 13b, and then by the thermosensitive element 15, and the heater 14 controls the heat. However, although the heater 14 radiates heat to the crystal elements 13a and 13b, the heat is transmitted to the crystal elements 13a and 13b mainly through the substrate 12 and a fixing member 16. Therefore, it takes quite a long time to cancel the temperature change received by the crystal elements 13a and 13b. 
Thus, if a temperature difference can be easily made between the crystal elements 13a and 13b and the oscillation circuit and there is poor response for stabilizing the temperature of the crystal elements 13a and 13b, then the oscillation frequency from the crystal oscillator becomes unstable, and if an external temperature is different, the rate of the temperature change of each unit is also different. Therefore, when a practical operation is performed, the crystal oscillator cannot be guaranteed as a stable device.
Furthermore, since the above-mentioned structure is generated based on the concept of controlling the temperature of the crystal elements 13a and 13b directly by the heater 14, the crystal elements 13a and 13b are uncovered. Since the uncovered crystal elements 13a and 13b, heater 14, thermosensitive element 15, etc. are arranged in the same package 11, it is necessary to remove the particles and volatile components spread from each part and adhere to the crystal elements 13a and 13b during the production process. As a result, before the parts are enclosed in the package 11, the crystal elements 13a and 13b have to be treated in a cleaning process by complicatedly combining UV-cleaning, plasma-cleaning, etc. with each part assembled on the base 11a, thus requiring the additional cost for the process.
[Patent Document 1] U.S. Pat. No. 5,917,272, FIGS. 1 through 5